The present invention relates to containers for storing fluids at a pressure higher than atmospheric pressure, of the kind comprising an outer shell which is formed by winding fibres of high specific mechanical strength impregnated with thermosetting resins and which resists the mechanical stresses set up by the fluids, and an inner wall of metallic material which forms an inner lining for the said shell and which provides a seal. Hereinafter, such containers will be referred to as "of the kind described".
Containers of the kind described are used for storing and transporting fluids of all kinds, be they liquid or gaseous and corrosive or non-corrosive at pressures which are generally high, that is to say greater than four bars. Their method of construction means that they are extremely light, which causes them to be preferred, in many applications, to containers made entirely of metal, whose dead weight is excessive.
The outer shell is made of a fibrous material, such as fibres of glass, polyamide, carbon, graphite, metal or boron, which are wound in circumferential or helical coils.
The object of the thermosetting resin with which the fibres are impregnated is to connect them together and it may be formed by a synthetic resin such as phenol-formaldehyde, polyester or epoxy resin. This shell, which forms a reinforcing structure to enable the containers to withstand the pressure of the fluid, is capable of withstanding an elastic deformation of 2 to 3% before fracture.
When the container is in use, the metal sealing wall, or liner, which is situated inside this shell is subjected to successive filling and emptying operations, that is to say to pressurisation and depressurisation cycles, and to the mechanical stresses which result. In certain present day containers, this wall is made of an aluminium alloy or of stainless steel. Although these metal walls, in contrast to thermo-plastics liners, have the advantage of being compatible with the majority of fluids, and in particular with oxygen, they are capable of withstanding only a very small amount of elastic deformation, i.e. less than 0.5%, that is to say an amount which is appreciably less than the outer shell can withstand. The inner wall is thus unable to follow deformation of the outer shell because it soon reaches the zone of plastic deformation. However, even when the shell is stressed to only a third of its breaking strength, the inner wall is already subject to excessive deformation which soon causes it to cold-flow and cracks to appear and finally the wall to fracture. In fact, the resistance which containers of the kind described have to stresses due to the periodic variations in pressure which occur during the pressurising and depressurising cycles thereof, is highly inadequate. In fact, their useful life does not generally exceed a thousand to two thousand such cycles.
Any increase in the thickness of the inner wall or the outer shell, with the object of restricting deformation, results in an increase in the weight of the container, which becomes as heavy as if it were made entirely of aluminium or steel.
Various solutions have been proposed to the problem of increasing the ability of the liner to deform.
One of these methods of manufacture, which is described in French patent application No. 2,137,976, consists in forming a layer to distribute the strain in the dome-shaped region of the container in order to reduce the area subject to high stress. In fact, containers constructed by this method soon show cracks and buckling in the region of the domes.
Another solution which is described in French patent specification No. 1,342,496, consists in providing a corrugated inner wall. Such a construction is expensive and does not substantially increase the useful life of the containers.
The disadvantages of the solutions proposed hitherto have led inventors to study more closely the knowledge so far acquired concerning the material forming the liner and the stresses which exist in this material.
It is known that many metallic materials, referred to as "super-elastic materials", have the characteristic of undergoing a transformation of the martensitic type which results in considerable changes in their physical properties.
This transformation may occur as a result of a change in the temperature of the material in the absence of mechanical stress, or as a result of mechanical stress exerted on the said material at a constant temperature. With certain metallic materials such as steels, when a martensitic transformation takes place at a constant temperature as a result of mechanical stress it is irreversible. With other materials on the other hand, this transformation of the martensitic type as a result of stress is reversible if the temperature at which the stress is exerted is suitably selected.
The temperature at which a structure of the martensitic type begins to appear, under no stress, when temperature decreases is generally referred to as the martensitic starting temperature M.sub.s. The M.sub.s temperature thus constitutes a point of change in crystalline structure, the material passing from a phase which is stable at high temperature (the .beta. phase for many alloys) to the martensitic phase, which endows the material with a particular capacity for deforming elastically termed "super-elasticity". When stress (traction or compression) is exerted on the material, the temperature at which a phase of the martensitic type begins to appear alters and increases with the increase in the said stress.
The martensitic transformation which thus occurs under the prompting of stress results in the metallic material having a capacity for reversible extension of more than 1%, which leads to such materials being used to produce the inner walls of pressurised containers.
One object of the invention is to provide a satisfactory solution to the problem of elastic deformation of the inner wall of containers of the kind described, and provide containers whose useful life is longer than that of containers known hitherto.